1. Field of the Invention
The present invention relates to a fall detection device detecting, on the basis of sensing an acceleration, whether or not a device is in a falling state, and to a magnetic disk device and a portable electronic apparatus having the same.
2. Description of the Related Art
As examples of prior art, Japanese Unexamined Patent Application Publication No. 2005-147899 and Japanese Patent No. 3441668 disclose devices with detection of falling states.
FIG. 1 illustrates the structure of the device of Japanese Unexamined Patent Application Publication No. 2005-147899. The fall detection device has an acceleration sensor 10, a differentiation circuit 42 for differentiating the acceleration detection signal and outputting a differentiation signal, a first comparator 44 for determining whether or not the differentiation signal reaches a first given threshold value, a second comparator 46 for determining whether or not the differentiation signal reaches a second given threshold value higher than the first threshold value, a voltage detector 50, and a processing circuit 48.
When an output signal of the acceleration sensor 10 reaches a predetermined set value, and the state is maintained for a given period of time, the fall detection device illustrated in FIG. 1 detects when the current state is a first processing state when the differentiation signal reaches the first threshold value. The fall detection device also detects when the current state is a second processing state when the differentiation signal reaches the second threshold value. Thus, in the first processing state, for example, a safety action is controlled to suspend a recording operation when a magnetic head of a hard disk device is recording. When detecting the second processing state, for example, the hard disk device is controlled to be in a much safer state.
The fall detection device of Japanese Patent No. 3441668 has an acceleration sensor and a fall determination processing unit and is configured such that both an acceleration and a differentiation value of the acceleration are simultaneously used for the determination.
In the fall detection device of Japanese Unexamined Patent Application Publication No. 2005-147899, it is necessary to simultaneously use two threshold values for determining whether or not a device is falling, which complicates the determination processing. Therefore, particularly when the determination is performed by software, the arithmetic load on CPU becomes large, causing a problem of prolonged processing time.
In Japanese Unexamined Patent Application Publication No. 2005-147899, although a falling state or a state where an impact has been applied can be individually determined, a state where the start of a fall is suspected (i.e., a fall prediction) cannot be detected, resulting in a problem in that a measure against the fall cannot be taken at an early stage.
In Japanese Unexamined Patent Application Publication No. 2005-147899, an acceleration is used for the determination. In general, a detected acceleration value of the acceleration sensor contains an offset, and thus an offset adjustment of the acceleration sensor is required.
When a device falls while rotating, for example, an acceleration due to the centrifugal force of the rotation is added. Thus, a detected acceleration value is likely to be influenced by the centrifugal force of the rotation. Thus, there is also a possibility that a fall is not appropriately determined when a device falls while rotating.
In the fall detection device of Japanese Patent No. 3441668, both an acceleration and a differentiation value of the acceleration are simultaneously used for the determination, and thus the determination processing is complicated. Thus, problems similarly as in the case of Japanese Unexamined Patent Application Publication No. 2005-147899 arise. Moreover, a fall cannot be predicted.